Practical methods of performing tracking servo for an optical disk include a differential push-pull (DPP) method and a differential phase detection (DPD) method. The DPP method uses a diffraction at a boundary between a land and a groove formed on a surface of an optical disk, in which an optical beam can be focused on a center of the land or the groove. Meanwhile, the DPD method uses a diffraction by a code (a pit or a recording mark) recorded on a recording layer, in which an optical beam can be focused on a center of a code string.
In the DPP method, an offset is generated due to a lens shift of an objective lens. Therefore, a three beam method is employed to cancel the offset by dividing the optical beam into three beams including a zeroth-order diffracted light and ±(plus-minus) first-order diffracted lights. Meanwhile, in the DPD method, a control is performed by detecting a phase difference between signals. Therefore, the offset caused by the lens shift of the objective lens gives substantially no problems, and a single beam method is employed for the DPD method.
When reproducing a ROM or a recorded optical disk, because a code is already recorded on a recording layer, it is possible to perform tracking servo by the DPD method at least in a recorded area (see, for example, Japanese Patent Application Laid-open No. 2005-293637 and Japanese Patent Application Laid-open No. 2002-74687, hereinafter, “Patent Document 1” and “Patent Document 2”, respectively). However, because the DPD method cannot be used in a non-recorded area and an irradiation point may come into a non-recorded area when reproducing a disk, the DPP method is conventionally used when reproducing an optical disk having a land and a groove.